Fire Codes for buildings in most States require that one and two story dwellings maintain and often upgrade the alarm systems by interconnecting their smoke alarms and CO detectors for simultaneous operation. After interconnection, when one alarm sensor detects a hazard at one end of the house, all other installed alarm sensors, even ones located at the other end of a house, as well as each bedroom, are energized simultaneously and begin to emit their alarm sound. (FIG. 3)
Alarm interconnection has been proven to give people more time to escape from a structural fire. That extra time results in the saving of lives and property in a far greater proportion than when interconnection is not used.
The conventional method of accomplishing the necessary interconnection is to install each device with a third electrical wire connection 6. Two wires, white 6W and black 6B, provide the commercial power, such as 120 VAC 60 Hz power in the United States, or other commercial power, such as 230 VAC 50 Hz found in other countries.
A third trigger wire, usually red, is normally strung between alarms and is employed for interconnecting the low voltage signal needed to activate the other alarms installed within the building. This is typically a standard 9 VDC. Most United States Building and Fire Codes require this form of alarm interconnection in all new construction. Property Maintenance Codes require existing homes to be upgraded in this manner when and where it is feasible. When a fire or CO alarm actuates, it shorts this 9 VDC to its yellow alarm wire, which is conductively connected to the structure's red alarm wire system.